Tip: cat body language is worth emulating, sometimes even with fellow humans!
A lot of the specifics of cat to cat communication are in the movement of the head, especially the degree to which the neck is exposed.
A happy squint followed by two quick nods upward, showing more neck, is a *lot* friendlier than just a happy squint. In white American culture, such nods are unheard of. We always nod downward, concealing the neck.
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Exposing the neck shows trust but also respect: it shows (a) I know you are strong, but (b) I also know you're cool and know when to use it correctly. The movement shows awareness of the strength, the exposed neck shows trust and respect. "I'd let her step on me" energy, even.
Cats can have entire conversations with each other, just by moving their heads. It's really impressive to watch.